ArrayBlockingQueue是JDK1.5開(kāi)始concurrent包中提供的并發(fā)工具類，是一個(gè)基于數(shù)組的有界的先進(jìn)先出隊(duì)列，如果加入元素時(shí)，數(shù)組已滿，或數(shù)組為空時(shí)，取元素都會(huì)阻塞當(dāng)前線程，是一個(gè)典型的生產(chǎn)者消費(fèi)者模式。類似的類還有LinkedBlockingQueue，PriorityBlockingQueue。

繼承關(guān)系

public class ArrayBlockingQueue extends AbstractQueue

implements BlockingQueue, java.io.Serializable

public interface BlockingQueue extends Queue

核心成員變量

final Object[] items; //保存元素

/** items index for next take, poll, peek or remove */

int takeIndex; //指向隊(duì)頭

/** items index for next put, offer, or add */

int putIndex; //指向隊(duì)尾

/** Number of elements in the queue */

int count; //隊(duì)中元素計(jì)數(shù)

/*

* Concurrency control uses the classic two-condition algorithm

* found in any textbook.

*/

/** Main lock guarding all access */

final ReentrantLock lock; //可重入鎖

/** Condition for waiting takes */

private final Condition notEmpty; //條件變量

/** Condition for waiting puts */

private final Condition notFull; //條件變量

構(gòu)造方法

public ArrayBlockingQueue(int capacity, boolean fair) {

if (capacity <= 0)

throw new IllegalArgumentException();

this.items = new Object[capacity]; //初始化指定大小的有界隊(duì)列

lock = new ReentrantLock(fair); //fair指示該鎖是否是公平的

notEmpty = lock.newCondition();

notFull = lock.newCondition();

}

添加元素:put方法

public void put(E e) throws InterruptedException {

checkNotNull(e);

final ReentrantLock lock = this.lock;

lock.lockInterruptibly();

try {

while (count == items.length)

notFull.await(); //添加元素時(shí)，如果已滿，則阻塞當(dāng)前線程，等待在notFull這個(gè)條件變量上，等待notFull調(diào)用signal喚醒

enqueue(e); //此時(shí)隊(duì)列肯定未滿

} finally {

lock.unlock();

}

}

private void enqueue(E x) {

// assert lock.getHoldCount() == 1;

// assert items[putIndex] == null;

final Object[] items = this.items;

items[putIndex] = x; //將元素加入隊(duì)尾

if (++putIndex == items.length) //將putIndex加1，如果到達(dá)數(shù)組尾部，則從0開(kāi)始

putIndex = 0;

count++; //增加計(jì)數(shù)

notEmpty.signal(); //增加一個(gè)元素后，如果有線程等待在noEmpty條件變量上，通知并喚醒一種一個(gè)線程。

}

put方法首先判斷隊(duì)列是否已滿，如果已滿，則阻塞當(dāng)前線程，等待在notFull這個(gè)條件變量上，等待notFull調(diào)用signal喚醒。當(dāng)隊(duì)列非空時(shí)，將該元素加入隊(duì)尾，增加元素計(jì)數(shù)，喚醒因?yàn)檫M(jìn)行去操作時(shí)數(shù)組為空而等待在notEmpty上的線程，通知它此時(shí)隊(duì)列已經(jīng)有元素了，你可以進(jìn)行取操作了。

取元素:take方法

public E take() throws InterruptedException {

final ReentrantLock lock = this.lock;

lock.lockInterruptibly();

try {

while (count == 0)

notEmpty.await(); //取元素但是隊(duì)列為空時(shí)，阻塞線程，等待在notEmpty上

return dequeue(); //此時(shí)，隊(duì)列肯定非空，進(jìn)行出隊(duì)操作

} finally {

lock.unlock();

}

}

private E dequeue() {

// assert lock.getHoldCount() == 1;

// assert items[takeIndex] != null;

final Object[] items = this.items;

@SuppressWarnings("unchecked")

E x = (E) items[takeIndex]; //獲取隊(duì)頭元素

items[takeIndex] = null;

if (++takeIndex == items.length)

takeIndex = 0;

count--; //計(jì)數(shù)減1

if (itrs != null)

itrs.elementDequeued();

notFull.signal(); //通知因?yàn)樘砑釉氐顷?duì)列已滿時(shí)而阻塞的線程，隊(duì)列此時(shí)可插入了

return x; //返回隊(duì)頭元素

}

take方法和put方法類似，先檢查隊(duì)列是否為空，隊(duì)列為空時(shí)，阻塞線程，等待在notEmpty上。當(dāng)隊(duì)列非空時(shí)，進(jìn)行出隊(duì)操作。同時(shí)還要通知因?yàn)樘砑釉氐顷?duì)列已滿時(shí)而阻塞的線程，隊(duì)列此時(shí)可插入了。

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